Administrator account cannot access all files/folders

Hi,
When I went to change the permissions and ownership of all files/folders on my G: drive (stand-alone partition), I was prevented from accessing some folders/files from the Administrator account. When I went to apply changes using the Properties->Security tab, I received a dialog message stating that I did not have access to such. I thought that the Administrator account was God-like in that it could apply permission changes across all file systems. I've also found that some files are restricted/owned by an account like S-2-1A-9274610829347... which has ? marks around it's name. I have been successful in adding my user id with permissions and deleting that unknown user from the Security tab, but, not with all files.

How can I set the ownership and permissions across all drives/folders/files on my G: drive to a specified user account, mine, not just the Administrator account? Do I have a virus on my computer that would prevent the Administrator account from having full access to all drives/folders/files?